Measurement of entrained and dissolved gases in process flow lines

ABSTRACT

A device for measurement of entrained and dissolved gas has a first module arranged in relation to a process line for providing a first signal containing information about a sensed entrained air/gas in a fluid or process mixture flowing in the process line at a process line pressure. The device features a combination of a bleed line, a second module and a third module. The bleed line is coupled to the process line for bleeding a portion of the fluid or process mixture from the process line at a bleed line pressure that is lower than the process pressure. The second module is arranged in relation to the bleed line, for providing a second signal containing information about a sensed bleed line entrained air/gas in the fluid or process mixture flowing in the bleed line. The third module responds to the first signal and the second signal, for providing a third signal containing information about a dissolved air/gas flowing in the process line based on a difference between the sensed entrained air/gas and the sensed bleed line entrained air/gas.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of U.S. Provisional PatentApplication No. 60/482,516 filed Jun. 24, 2003, (Attorney DocketCC-0604) U.S. Provisional Patent Application No. 60/441,652 filed Jan.22, 2003, (Attorney Docket CC-0585); U.S. Provisional Patent ApplicationNo. 60/441,395 filed Jan. 21, 2003, (Attorney Docket CC-0581); which areall incorporated herein by reference.

BACKGROUND OF THE INVENTION

[0002] 1. Technical Field

[0003] The present invention generally relates to a device for measuringdissolved air in a fluid or process mixture flowing in a process line.

[0004] 2. Description of Related Art

[0005] Monitoring levels of entrained and dissolved gases is desirablein many industrial processes. For example, entrained and dissolve gasesin the approach system of paper making machines are often problematic,leading to a wide variety of problems, including flow line pulsations,pin-holes in the produced paper, reduced paper sheet strength, andexcessive build-up of aerobic growths.

[0006] Entrained gases are gases that exist in a gaseous form, mixed inthe process fluid. For many industrial applications with small, lessthan ˜20% gas fraction by volume, the gas is typically in the form ofsmall bubbles contained in a liquid continuous mixture. Entrained gasesexist as either free bubbles moving within the stock or as bound (orresidual) air that is adhered to the fiber. In either cases, entrainedair can generally be detected by monitoring the compressibility of themixture and correlating the compressibility to volumetric percentage ofentrained air.

[0007] Dissolved gases are dissolved within the mixture on a molecularlevel. While in the solution, dissolved gases pose few operationproblems. Typically dissolved gases have a negligible effect on thecompressibility of the mixture. Thus, dissolved gases are difficult todetect via compressibility measurements.

[0008] Although dissolved gases are typically not problematic whiledissolved, problems arise when dissolved gases come out of a solution asa result of either decreases in pressure or increases in temperature.One example of this is in pressurized head boxes on paper machines wherethe pressure drop associated with spraying the pulp/water mixture on tothe paper machine can cause dissolved gases to come out of the solutionand form entrained air.

[0009] Thus, to accurately monitor problems associated with entrainedand dissolve gases, it is desirable to be able to measure bothquantities.

SUMMARY OF THE INVENTION

[0010] In its broadest sense, the present invention provides a new andunique device having a first module arranged in relation to a processline for providing a first signal containing information about a sensedentrained air/gas in a fluid or process mixture flowing in the processline at a process line pressure. The device features a combination of ableed line, a second module and a third module. The bleed line iscoupled to the process line for bleeding a portion of the fluid orprocess mixture from the process line at a bleed line pressure that islower than the process pressure. The second module is arranged inrelation to the bleed line, for providing a second signal containinginformation about a sensed bleed line entrained air/gas in the fluid orprocess mixture flowing in the bleed line. The third module responds tothe first signal and the second signal, for providing a third signalcontaining information about a dissolved air/gas flowing in the processline based on a difference between the sensed entrained air/gas and thesensed bleed line entrained air/gas.

[0011] In one embodiment, the first module is a primary process lineentrained air measurement module that includes an array of sensors thatmeasures the speed of sound propagating through the fluid or processmixture flowing within the process line and determines the entrained airbased on a measurement using the speed of sound. The second module is ableed line entrained air measurement module that also includes an arrayof sensors that measures the speed of sound propagating through thefluid or process mixture flowing within the bleed line and determinesthe bleed line entrained air based on a measurement using the speed ofsound. The third module is a dissolved air determination processormodule that processes the first and second signals and provides thethird signal containing information about a dissolved air/gas flowing inthe process line.

[0012] The device also includes a bleed line control module forcontrolling the bleeding off of the portion of the fluid from theprocess line via a bleed valve and the reinjection of said portion backto the process line via a boost pump, and also includes a controllermodule for controlling and coordinating the operation of the first,second and third modules, as well as a bleed line control thatcommunicates with the bleed valve and the boost pump.

[0013] In operation, the device according to the present inventionmeasures dissolved gases at an operating pressure by measuring entrainedgases present in a process line once the fluid or process mixture isexpanded to ambient (or other known and relevant) pressure. Thismeasurement is performed using a small amount of process mixturebled-off, either continuously or periodically, from the process. Thebled-off process fluid can be recirculated or, via a boost pump,re-pressurized and reinjected. The bleed line and flow rates may besized to minimize the amount of stock bleed off while maintainedsufficiently high flow rates to maintain sufficiently homogenous flowwithin the bled-off liquid test section (i.e. minimize slip) such thatthe measured gas volume fraction within the bleed line is indeedrepresentative of the amount of gas dissolved in the process fluid.Maintaining sufficiently high velocities avoids problems associated withstratification of the mixture and the problems associated with eitherthe liquid of gas phases “holding up” in the process pipe. For mostmixtures of liquids and gases at or near ambient pressures, flowvelocities of several feet per second through the line are sufficient.

[0014] The process of the throttling of the process fluid to the reducedpressure provides sufficient noise to perform a sonar-based speedmeasurement.

[0015] The present invention also provides a method for measuring theentrained gas fraction at two relevant pressures, and thus providespractical measurement of the amount of both entrained and dissolvedgases contained in the process fluid at the process operatingconditions.

[0016] The foregoing and other objects, features and advantages of thepresent invention will become more apparent in light of the followingdetailed description of exemplary embodiments thereof.

BRIEF DESCRIPTION OF THE DRAWING

[0017] The drawing, not drawn to scale, includes the following Figures:

[0018]FIG. 1 is a block diagram of a device for measurement of entrainedand dissolved gases that is the subject matter of the present invention.

[0019]FIG. 2 is a schematic of the device shown in FIG. 1.

[0020]FIG. 3 is an overview of the system according to the presentinvention.

[0021]FIG. 4 is a graph of a gas volume fraction (GVF) between 0.00001and 0.1 versus a mixture sound speed in meters per sec (m/s).

[0022]FIG. 5 is a graph of a gas volume fraction (GVF) between 0.0 and0.1 versus a mixture sound speed in meters per sec.

[0023]FIG. 7 is a block diagram of an apparatus for measuring entrainedair in a fluid flowing within a pipe, such as a bleed line and primaryprocess line, in accordance with the present invention.

[0024]FIG. 8 is a block diagram of another embodiment of an apparatusfor measuring entrained air in a fluid flowing within a pipe, such as ableed line and primary process line, in accordance with the presentinvention.

BEST MODE FOR CARRYING OUT THE INVENTION

[0025]FIGS. 1 and 2 show a schematic and block diagram of a devicegenerally indicated as 10 for measurement of entrained and dissolvedgases in a fluid or process mixture generally indicated as 11 flowing ina primary process line 12 having a given process pressure.

[0026] In FIG. 1, the device 10 includes a first, or primary processline, entrained air measurement module 14, a bleed line 16 (see alsoFIG. 2), a second, or bleed line, entrained air measurement module 18and a dissolved air/gas determination processor module 20. The primaryprocess line entrained air measurement module 14 is arranged in relationto the primary process line 12, for sensing entrained air in the fluidor process mixture 11 and providing a first entrained air measurementmodule signal via line 14 a containing information about sensed primaryprocess line entrained air. As best shown in FIG. 2, the bleed line 16is coupled to the primary process line 12 for bleeding fluid or processmixture from the primary process line 12 at a bleed line pressure thatis lower than the process pressure, for example, at ambient pressure. Asbest shown in FIG. 1, the second entrained air measurement module 18 isarranged in relation to the bleed line 16, for sensing entrained air inthe fluid or process mixture in the bleed line, and providing a secondentrained air measurement module signal via line 18 a containinginformation about sensed bleed line entrained air. In FIG. 1, thedissolved air/gas determination processor module 20 responds to thefirst entrained air measurement module signal along line 14 a and thesecond entrained air measurement module signal along line 18 a, eachsignal being received via a controller module 22 as shown and discussedbelow; determines dissolved air/gas in the fluid or process mixtureflowing in the primary process line based on a difference between thesensed primary process line entrained air and the sensed bleed lineentrained air; and provides a dissolved air/gas determination processormodule signal containing information about the same.

[0027] The controller module 22 controls and coordinates the operationof the modules 14, 18, 20 and 32. As shown, the signals along lines 14a, 18 a are provided directly to the controller module 22, although thescope of the invention is intended to include embodiments in which thesignals along lines 14 a, 18 a are provided directly to the dissolvedair/gas determination processor module 20.

[0028] The primary process line entrained air measurement module 14includes an array of sensors 24 shown in FIG. 2 that measures the speedof sound propagating through the fluid or process mixture flowing withinthe process line 12 and determines the entrained air based on ameasurement using the speed of sound, as will be described in greaterdetail hereinafter.

[0029] Similarly, the bleed line entrained air measurement module 18includes a corresponding array of sensors 26 that measures the speed ofsound propagating through the fluid or process mixture 11 flowing withinthe bleed line 16 and determines the bleed line entrained air based on ameasurement using the speed of sound.

[0030] The bleed line 16 has a bleed valve 28 for bleeding the fluid orprocess mixture 11 into the bleed line 16. The bleed line 16 isre-coupled to the primary process line 12 via a boost pump 30 torecirculate the fluid or process mixture bled therefrom. The scope ofthe invention is not intended to be limited to the type or kind of bleedvalve or boost pump used. The invention is shown and described inrelation to a closed loop system; however, the scope of the invention isintended to include an open loop system in which the media from thebleed line is not returned to the process line.

[0031] The device 10 also includes a bleed line control module forcontrolling the bleeding off of the portion of the fluid or processmixture from the process line via the bleed valve 28 and the reinjectionof the same back to the process line 12 via the boost pump 30.

[0032] The modules 14, 18, 20, 22, 32 may be implemented using hardware,software, or a combination thereof. The scope of the invention is notintended to be limited to any particular implementation thereof. Forexample, a typical software implementation may include using amicroprocessor architecture having a microprocessor, a random accessmemory (RAM), a read only memory (ROM), input/output devices and acontrol, address and databus for connecting the same.

[0033] Although the invention is described in relation to measuring orsensing entrained air in a fluid or process mixture using an array ofsensors, the scope of the invention is intended to include other ways ofmeasuring or sensing entrained air either known now or developed in thefuture. Moreover. although the invention is described in relation tousing an array of sensors to determine the speed of sound, the scope ofthe invention is intended to include other ways of measuring the speedof sound either known now or developed in the future.

Entrained Gas Measurement

[0034] The present invention uses the speed at which sound propagateswithin a conduit to measure entrained air in slurries. This approach maybe used with any technique that measures the sound speed of a fluid orprocess mixture. However, it is particularly synergistic with sonarbased volumetric flow meters such as described in aforementioned U.S.patent application Ser. No. 10/007,736 (CiDRA's Docket No. CC-0122A), inthat the sound speed measurement, and thus gas volume fractionmeasurement, can be accomplished using the same hardware as thatrequired for the volumetric flow measurement. It should be noted,however, that the gas volume fraction (GVF) measurement could beperformed independently of a volumetric flow measurement, and would haveutility as an important process measurement in isolation or inconjunction with other process measurements.

[0035] Firstly, the sound speed may be measured as described inaforementioned U.S. patent application Ser. No. 09/344,094 (CiDRA'sDocket No. CC-0066A), U.S. patent application Ser. No. 10/007,749(CiDRA's Docket No. CC-0066B), U.S. patent application Ser. No.10/349,716 filed Jan. 23, 2003 (Cidra's Docket No. CC-0579) and/or U.S.patent application Ser. No. 10/376,427 filed Feb. 26, 2003 (Cidra'sDocket No. CC-0596), all incorporated herein by reference, using anarray of unsteady pressure transducers. For a two component mixture,utilizing relations described in U.S. patent application Ser. No.09/344,094 (CiDRA's Docket No. CC-0066A) and/or U.S. patent applicationSer. No. 10/007,749 (CiDRA's Docket No. CC-0066B), knowledge of thedensity and sound speed of the two components and the complianceproperties of the conduit or pipe, the measured sound speed can be usedto determine the volumetric phase fraction of the two components.

[0036] The sound speed of a mixture can be related to volumetric phasefraction (φ_(i)) of the components and the sound speed (a) and densities(ρ) of the component through the Wood equation, where$\frac{1}{\rho_{mix}a_{{mix}\quad \infty}^{2}} = {{\sum\limits_{i = 1}^{N}\quad {\frac{\varphi_{i}}{\rho_{i}a_{i}^{2}}\quad {where}\quad \rho_{mix}}} = {\sum\limits_{i = 1}^{N}{\rho_{i}\varphi_{i}}}}$

[0037] One dimensional compression waves propagating within a fluidcontained within a conduit exert an unsteady internal pressure loadingon the conduit. The degree to which the conduit displaces as a result ofthe unsteady pressure loading influences the speed of propagation of thecompression wave. The relationship among the infinite domain speed ofsound and density of a fluid; the elastic modulus (E), thickness (t),and radius (R) of a vacuum-backed cylindrical conduit; and the effectivepropagation velocity (a_(eff)) for one dimensional compression is givenby the following expression:${a\quad e_{ff}} = \frac{1}{\sqrt{\frac{1}{a_{{mix}\quad \infty}^{2}} + {\rho_{mix}\frac{2R}{E\quad t}}}}$

[0038] Note: “vacuum backed” as used herein refers to a situation inwhich the fluid surrounding the conduit externally has negligibleacoustic impedance compared to that of the fluid internal to the pipe.For example, meter containing a typical water and pulp slurry immersedin air at standard atmospheric conditions satisfies this condition andcan be considered “vacuum-backed”.

[0039] For paper and pulp slurries, the conditions are such that forslurries with non-negligible amounts of entrained gas, say <0.01%, thecompliance of standard industrial piping (Schedule 10 or 40 steel pipe)is typically negligible compared to that of the entrained air.

[0040]FIGS. 4 and 5 show the relationship between sound speed andentrained air for slurries with pulp contents representative of therange used in the paper and pulp industry. Referring to FIG. 4, twoslurry consistencies are shown; representing the lower limit, a purewater mixture is considered, and representing the higher end ofconsistencies, a 5% pulp/95% water slurry is considered. Since theeffect of entrained air on the sound speed of the mixture is highlysensitive to the compressibility of the entrained air, the effect of theentrained air is examined at two pressures, one at ambient representingthe lower limit of pressure, and one at 4 atmospheres representing atypical line pressure in a paper process. As shown, the consistency ofthe liquid slurry, i.e., the pulp content, has little effect on therelationship between entrained air volume fraction and mixture soundspeed. This indicates that an entrained air measurement could beaccurately performed, within 0.01% or so, with little or no knowledge ofthe consistency of the slurry. The chart does show a strong dependenceon line pressure. Physically, this effect is linked to thecompressibility of the air, and thus, this indicates that reasonableestimates of line pressure and temperature would be required toaccurately interpret mixture sound speed in terms of entrained air gasvolume fraction.

[0041]FIG. 4 also shows that for the region of interest, from roughly 1%entrained air to roughly 5% entrained air, mixture sound speeds(a_(mix)) are quite low compared to the liquid-only sound speeds. In theexample shown above, the sound speed of the pure water and the 5% pulpslurry were calculated, based on reasonable estimates of the constituentdensities and compressibilities, to be 1524 m/s and 1541 m/s,respectively. The sound speed of these mixtures with 1% to 5% entrainedair at typical operating pressure (1 atm to 4 atms) are on the order of100 n/sec. The implication of these low sound speeds is that the mixturesound speed could be accurately determined with an array of sensors,i.e. using the methodology described in aforementioned U.S. patentapplication Ser. No. 09/344,094 (CiDRA's Docket No. CC-0066A), and/orU.S. patent application Ser. No. 10/007,749 (CiDRA's Docket No.CC-0066B), with an aperture that is similar, or identical, to an arrayof sensors that would be suitable to determine the convection velocity,using the methodology described in aforementioned U.S. patentapplication Ser. No. 10/007,736 (CiDRA's Docket No. CC-0122A), which isincorporated herein by reference.

[0042] A flow chart of the proposed measurement is shown in FIG. 3,where the inputs are the mixture of SOS, P and T are pressure andtemperature, respectively, and GVT air (gas volumetric flow of air) isprovided from the box “Entrained Air Volume Fraction” as an output andto the box “correct for void fraction of air” and Q mixture (volumetricflow of the mixture) is provided from the box “Total Mixture Flow Rate”as an output and to the box “correct for void fraction of air”.

[0043] Other information relating to the gas volume fraction in a fluidand the speed of sound (or sonic velocity) in the fluid, is described in“Fluid Mechanics and Measurements in two-phase flow Systems”,Institution of mechanical engineers, proceedings 1969-1970 Vol. 184 part3C, Sep. 24-25 1969, Birdcage Walk, Westminster, London S.W. 1, England.

[0044] Based on the above discussion, one may use a short length scaleaperture to measure the sound speed.

[0045] The characteristic acoustic length scale is: λ=c/f, where c isthe speed of sound in a mixture, f is frequency and λ is wavelength.

[0046] If Aperture=L and if L/λ is approx. constant.

[0047] Then Lwater/λwater=Lwater*f/C_(water)≈L_(GVF)*f/c_(GvF)

[0048] Therefore: L_(GVF)=Lwater (C_(GVF)/C_(water)); where GVF is gasvolume fraction.

[0049] Thus for SOS of water (Cwater=5,000 ft/sec) , and SOS of the Gasvolume fraction (C GVF=500 ft/sec) and a length aperture of L water=5 ft(which we have shown is sufficient to accurately measure the SOS ofwater), the length aperture for a gas volume fraction L_(GVF) would beabout 0.5 feet.

[0050] Note that this entrained air or gas volume fraction measurementGVFair may be used with any flow meter or consistency meter to correctfor errors introduced into a measurement by entrained air. Inparticular, an electromagnetic flow meter will show an error whenentrained air exists in the mixture. The present invention may be usedto correct for this error. In addition, a consistency meter will show anerror when entrained air exists in the mixture. The present inventionmay be used to correct for this error.

[0051] The scope of the invention is also intended to include usingother models and corrections for determining entrained air in a fluidthat may be used to compensate for gas volume fraction.

[0052] As shown in FIG. 3, the sonar meter measures the speed at whichacoustic wave propagating in the process piping to determine the amountof entrained air in the process line. The acoustic wave can be generatedby a pump or other device disposed in the piping system, or generatedsimply by the mixture/fluid flowing through the pipe, all of whichprovide a passive acoustic source. Alternatively, the sonar flow meterincludes an active acoustic source that injects an acoustic wave intothe flow such as by compressing, vibrating and/or tapping the pipe, toname a few examples.

[0053] The connection between speed of sound of a two-phase mixture andphase fraction is well established for mixtures in which the wavelengthof the sound is significantly larger than any inhomogenieities, i.e.bubbles, in the flow.

[0054] The mixing rule essentially states that the compressibility of amixture (1/(ρ a²)) is the volumetrically-weighted average of thecompressibilities of the components. For gas/liquid mixtures at pressureand temperatures typical of paper and pulp industry, the compressibilityof gas phase is orders of magnitudes greater than that of the liquid.Thus, the compressibility of the gas phase and the density of the liquidphase primarily determine mixture sound speed, and as such, it isnecessary to have a good estimate of process pressure to interpretmixture sound speed in terms of volumetric fraction of entrained air.The effect of process pressure on the relationship between sound speedand entrained air volume fraction is shown in FIG. 4.

[0055] Conversely, however, detailed knowledge of the liquid/slurry isnot required for entrained air measurement. Variations in liquid densityand compressibility with changes in consistency have a negligible effecton mixture sound speed compared to the presence of entrained air. FIG. 5shows the mixture sound speed as a function of entrained air volumefraction for two slurries, one with 0% wood fiber and the other with 5%wood fiber by volume. As shown, the relationship between mixture soundspeed and gas volume fraction is essentially indistinguishable for thetwo slurries. Furthermore, mixture sound speed is shown to an excellentindicator of gas volume fraction, especially for the trace to moderateamounts of entrained air, from 0 to 5% by volume, typically encounteredin the paper and pulp industry.

Speed of Sound Measurement

[0056] As mentioned earlier, the relationship between mixture soundspeed and entrained air in bubbly liquids is well established. However,as will be developed below, in bubbly flows, these relations are onlyapplicable for the propagation of relatively low frequency, longwavelength sound. While this restriction does not present anysignificant obstacles for the sonar meter, it does present significantchallenges to ultrasonic sound speed measurement devices.

[0057] Ultrasonic meters typically operate in 100 Khz to several Mhzfrequency range. For these meters, entrained air bubbles have lengthscales on the same order as the acoustic waves generated by theultrasonic meters. The posed several problems. Firstly, the bubblesscatter the ultrasonic waves, impairing the ability of the ultrasonicmeter to perform a sound speed measurement. Also, ultrasonic meters relyon information derived from only a small fraction of the cross sectionalarea of the pipe to be representative of the entire cross section, anassumption that breaks down for flows with inhogenieties on the samelength scale as the ultrasonic wavelength.

[0058] Sonar flow meters use an approach developed and commercializedspecifically for multiphase flow measurement in the oil and gasindustry. Sonar meters measure the propagation velocity of operationallygenerated sound in the ˜100 to 1000 Hz frequency range. In thisfrequency range, sound propagates as a one-dimensional wave using theprocess pipe as a waveguide. The wavelength of sound in this frequencyrange (>1 m) is typically several orders of magnitude larger than thelength scale of the any bubbles. The long wavelength acoustics propagatethrough the bubbles unimpeded, providing a robust and representativemeasure of the volumetrically averaged properties of the flow.

[0059] For the sound speed measurement, the sonar flow meter utilizessimilar processing algorithms as those employed for the volumetric flowmeasurement. As with convective disturbances, the temporal and spatialfrequency content of sound propagating within the process piping isrelated through a dispersion relationship.

k=ω/a _(mix)

[0060] As before, k is the wave number, defined as k=2π/λ, ω is thetemporal frequency in rad/sec, and a_(mix) is the speed at which soundpropagates within the process piping. Unlike disturbances which convectwith the flow, however, sound generally propagates in both directions,with and against the mean flow. For these cases, the acoustic power islocated along two acoustic ridges, one for the sound traveling with theflow at a speed of a_(mix)+V_(mix) and one for the sound travelingagainst the flow at a speed of a_(mix)−V_(mix).

[0061]FIG. 6 shows a k-ω plot generated for acoustic sound fieldrecorded from water flowing at a rate of 240 gpm containing ˜2%entrained air by volume in a 3 in, schedule 10, stainless steel pipe.The k-ω plot was constructed using data from an array of strain-basedsensors attached to the outside of the pipe. Two acoustic ridges areclearly evident. Based on the slopes of the acoustic ridges, the soundspeed for this for this mixture was 330 ft/sec (100 m/s), consistentwith that predicted by the Wood equation. Note that adding 2% air byvolume reduces the sound speed of the bubbly mixture to less than 10% ofthe sound speed of single phase water. FIG. 15 illustrates a schematicdrawing of one embodiment of the present invention. The apparatus 210includes a sensing device 16 comprising an array of pressure sensors (ortransducers) 18-21 spaced axially along the outer surface 22 of a pipe14, having a process flow propagating therein, similar to that describedhereinbefore. The pressure sensors measure the unsteady pressuresproduced by acoustical disturbances within the pipe, which areindicative of the SOS propagating through the mixture 12. The outputsignals (P₁-P_(N)) of the pressure sensors 18-21 are provided to theprocessor 224, which processes the pressure measurement data anddetermines the speed of sound, gas volume fraction (GVF) and otherparameters of the flow as described hereinbefore.

[0062] In an embodiment of the present invention shown in FIG. 7, theapparatus 210, similar to the arrays 24,26 of FIG. 1, has at least twopressure sensors 218-221 disposed axially along the pipe 214 formeasuring the unsteady pressure P₁-P_(N) of the mixture 212 flowingtherethrough. The speed of sound propagating through the flow 212 isderived by interpreting the unsteady pressure field within the processpiping 214 (e.g., the bleed line 16 and primary process line 12) usingmultiple transducers displaced axially over ˜2 diameters in length. Theflow measurements can be performed using ported pressure transducers orclamp-on, strain-based sensors.

[0063] The apparatus 210 has the ability to measure the gas volumefraction and other parameters by determining the speed of sound ofacoustical disturbances or sound waves propagating through the flow 212using the array of pressure sensors 218-221.

[0064] Generally, the apparatus 210 measures unsteady pressures createdby acoustical disturbances propagating through the flow 212 to determinethe speed of sound (SOS) propagating through the flow. Knowing thepressure and/or temperature of the flow and the speed of sound of theacoustical disturbances, the processing unit 224 can determine the gasvolume fraction of the mixture, similar to that shown in U.S. patentapplication Ser. No. 10/349,716 (Cidra Docket No. CC-0579), filed Jan.21, 2003, U.S. patent application Ser. No. 10/376,427 (Cidra Docket No.CC-0596), filed Feb. 26, 2003, and U.S. Provisional Patent ApplicationSerial No. 60/528,802 (Cidra Docket No. CC-0685), filed Dec. 11, 2003which are all incorporated herein by reference.

[0065] The apparatus in FIG. 210 also contemplates providing one or moreacoustic sources 227 to enable the measurement of the speed of soundpropagating through the flow for instances of acoustically quiet flow.The acoustic sources may be disposed at the input end of output end ofthe array of sensors 218-221, or at both ends as shown. One shouldappreciate that in most instances the acoustics sources are notnecessary and the apparatus passively detects the acoustic ridgeprovided in the flow 212. The passive noise includes noise generated bypumps, valves, motors, and the turbulent mixture itself.

[0066] The apparatus 210 of the present invention may be configured andprogrammed to measure and process the detected unsteady pressuresP₁(t)-P_(N)(t) created by acoustic waves propagating through the mixtureto determine the SOS through the flow 212 in the pipe 214. One suchapparatus 310 is shown in FIG. 8 that measures the speed of sound (SOS)of one-dimensional sound waves propagating through the mixture todetermine the gas volume fraction o f the mixture. It is known thatsound propagates through various mediums at various speeds in suchfields as SONAR and RADAR fields. The speed of sound propagating throughthe pipe and mixture 212 may be determined using a number of knowntechniques, such as those set forth in U.S. patent application Ser. No.09/344,094, entitled “Fluid Parameter Measurement in Pipes UsingAcoustic Pressures”, filed Jun. 25, 1999, now U.S. Pat. No. 6,354,147;U.S. patent application Ser. No. 09/729,994, filed Dec. 4, 2002, nowU.S. Pat. No. 6,609,069; U.S. patent application Ser. No. 09/997,221,filed Nov. 28, 2001, now U.S. Pat. No. 6,587,798; and U.S. patentapplication Ser. No. 10/007,749, entitled “Fluid Parameter Measurementin Pipes Using Acoustic Pressures”, filed Nov. 7, 2001, each of whichare incorporated herein by reference.

[0067] In accordance with one embodiment of the present invention, thespeed of sound propagating through the mixture 212 is measured bypassively listening to the flow with an array of unsteady pressuresensors to determine the speed at which one-dimensional compressionwaves propagate through the mixture 212 contained within the pipe 214.

[0068] As shown in FIG. 8, an apparatus 310 embodying the presentinvention has an array of at least two acoustic pressure sensors115,116, located at three locations x₁,x₂ axially along the pipe 214.One will appreciate that the sensor array may include more than twopressure sensors as depicted by pressure sensors 117,118 at locationx₃,x_(N). The pressure generated by the acoustic waves may be measuredthrough pressure sensors 115-118. The pressure sensors 215-218 providepressure time-varying signals P₁(t),P₂(t),P₃(t),P_(N)(t) on lines120,121,122,123 to a signal processing unit 130 to known Fast FourierTransform (FFT) logics 126,127,128,129, respectively. The FFT logics126-129 calculate the Fourier transform of the time-based input signalsP₁(t)-P_(N)(t) and provide complex frequency domain (or frequency based)signals P₁(ω),P₂(ω),P₃(ω),P_(N)(ω) on lines 132,133,134,135 indicativeof the frequency content of the input signals. Instead of FFT's, anyother technique for obtaining the frequency domain characteristics ofthe signals P₁(t)-P_(N)(t), may be used. For example, the cross-spectraldensity and the power spectral density may be used to form a frequencydomain transfer functions (or frequency response or ratios) discussedhereinafter.

[0069] The frequency signals P₁(ω)-P_(N)(ω) are fed to an arrayprocessing unit 138 which provides a signal to line 40 indicative of thespeed of sound of the mixture a_(mix), discussed more hereinafter. Thea_(mix) signal is provided to an entrained gas processing unit 142,similar to the processing unit 225, which converts a_(mix) to a percentcomposition of a mixture and provides a gas volume fraction or %Compsignal to line 244 indicative thereof (as discussed hereinafter).

[0070] The data from the array of sensors 115-118 may be processed inany domain, including the frequency/spatial domain, the temporal/spatialdomain, the temporal/wave-number domain or the wave-number/frequency(k-_(ω)) domain. As such, any known array processing technique in any ofthese or other related domains may be used if desired, similar to thetechniques used in the fields of SONAR and RADAR.

[0071] One such technique of determining the speed of sound propagatingthrough the flow 212 is using array processing techniques to define anacoustic ridge in the k-ω plane as shown in FIG. 9. The slope of theacoustic ridge is indicative of the speed of sound propagating throughthe flow 212. This technique is similar to that described in U.S. Pat.No. 6,587,798 filed Nov. 28, 2001, titled “Method and System forDetermining The Speed of Sound in a Fluid Within a Conduit”, which isincorporated herein by reference. The speed of sound (SOS) is determinedby applying sonar arraying processing techniques to determine the speedat which the one dimensional acoustic waves propagate past the axialarray of unsteady pressure measurements distributed along the pipe 214.

[0072] The signal processor 224 performs a Fast Fourier Transform (FFT)of the time-based pressure signals P₁(t)-P_(N)(t) to convert thepressure signal into the frequency domain. The power of thefrequency-domain pressure signals are then determined and defined in thek-ω plane by using array processing algorithms (such as Capon and Musicalgorithms). The acoustic ridge in the k-ω plane, as shown in the k-ωplot of FIG. 6, is then determined. The speed of sound (SOS) isdetermined by measuring slope of the acoustic ridge. The gas volumefraction is then calculated or otherwise determined, as describedhereinafter.

[0073] The flow meter of the present invention uses known arrayprocessing techniques, in particular the Minimum Variance,Distortionless Response or other adaptive array processing techniques(MVDR, Music, or Capon technique), to identify pressure fluctuations,which convect with the materials flowing in a conduit and accuratelyascertain the velocity, and thus the flow rate, of said material. Theseprocessing techniques utilize the covariance between multiple sensors218-221 at a plurality of frequencies to identify signals that behaveaccording to a given assumed model; in the case of the apparatus 310, amodel, which represents pressure variations 220 convecting at a constantspeed across the pressure sensors comprising the flow meter monitoringhead 212.

[0074] Also, some or all of the functions within the processor 130 maybe implemented in software (using a microprocessor or computer) and/orfirmware, or may be implemented using analog and/or digital hardware,having sufficient memory, interfaces, and capacity to perform thefunctions described herein.

[0075] For certain types of pressure sensors, e.g., pipe strain sensors,accelerometers, velocity sensors or displacement sensors, discussedhereinafter, it may be desirable for the pipe 214 to exhibit a certainamount of pipe compliance.

[0076] The pressure sensors 218-221 described herein may be any type ofpressure sensor, capable of measuring the unsteady (or ac or dynamic)pressures within a pipe, such as piezoelectric, optical, capacitive,resistive (e.g., Wheatstone bridge), accelerometers (or geophones),velocity measuring devices, displacement measuring devices, etc. Ifoptical pressure sensors are used, the sensors 218-221 may be Bragggrating based pressure sensors, such as that described in U.S. patentapplication Ser. No. 08/925,598, entitled “High Sensitivity Fiber OpticPressure Sensor For Use In Harsh Environments”, filed Sep. 8, 1997, nowU.S. Pat. No. 6,016,702, which are incorporated herein by reference.Alternatively, the sensors 218-221 may be electrical or optical straingages attached to or embedded in the outer or inner wall of the pipewhich measure pipe wall strain, including microphones, hydrophones, orany other sensor capable of measuring the unsteady pressures within thepipe 214. In an embodiment of the present invention that utilizes fiberoptics as the pressure sensors 218-221 they may be connectedindividually or may be multiplexed along one or more optical fibersusing wavelength division multiplexing (WDM), time division multiplexing(TDM), or any other optical multiplexing techniques.

[0077] For any of the embodiments described herein, the pressuresensors, including electrical strain gages, optical fibers and/orgratings among others as described herein, may be attached to the pipeby adhesive, glue, epoxy, tape or other suitable attachment means toensure suitable contact between the sensor and the pipe 214. The sensorsmay alternatively be removable or permanently attached via knownmechanical techniques such as mechanical fastener, spring loaded,clamped, clam shell arrangement, strapping or other equivalents.Alternatively, the strain gages, including optical fibers and/orgratings, may be embedded in a composite pipe. If desired, for certainapplications, the gratings may be detached from (or strain oracoustically isolated from) the pipe 214 if desired.

[0078] It is also within the scope of the present invention that anyother strain sensing technique may be used to measure the variations instrain in the pipe, such as highly sensitive piezoelectric, electronicor electric, strain gages attached to or embedded in the pipe 214.

[0079] In certain embodiments of the present invention apiezo-electronic pressure transducer may be used as one or more of thepressure sensors 218-221 and it may measure the unsteady (or dynamic orac) pressure variations inside the pipe 214 by measuring the pressurelevels inside of the pipe 14. In an embodiment of the present inventionthe sensors 218-221 comprise pressure sensors manufactured by PCBPiezotronics. In one pressure sensor there are integrated circuitpiezoelectric voltage mode-type sensors that feature built-inmicroelectronic amplifiers, and convert the high-impedance charge into alow-impedance voltage output. Specifically, a Model 106B manufactured byPCB Piezotronics is used which is a high sensitivity, accelerationcompensated integrated circuit piezoelectric quartz pressure sensorsuitable for measuring low pressure acoustic phenomena in hydraulic andpneumatic systems. It has the unique capability to measure smallpressure changes of less than 0.001 psi under high static conditions.The 106B has a 300 mV/psi sensitivity and a resolution of 91 dB (0.0001psi).

[0080] The pressure sensors incorporate a built-in MOSFETmicroelectronic amplifier to convert the high-impedance charge outputinto a low-impedance voltage signal. The sensor is powered from aconstant-current source and can operate over long coaxial or ribboncable without signal degradation. The low-impedance voltage signal isnot affected by triboelectric cable noise or insulationresistance-degrading contaminants. Power to operate integrated circuitpiezoelectric sensors generally takes the form of a low-cost, 24 to 27VDC, 2 to 20 mA constant-current supply. A data acquisition system ofthe present invention may incorporate constant-current power fordirectly powering integrated circuit piezoelectric sensors.

[0081] Most piezoelectric pressure sensors are constructed with eithercompression mode quartz crystals preloaded in a rigid housing, orunconstrained tourmaline crystals. These designs give the sensorsmicrosecond response times and resonant frequencies in the hundreds ofkHz, with minimal overshoot or ringing. Small diaphragm diameters ensurespatial resolution of narrow shock waves.

[0082] The output characteristic of piezoelectric pressure sensorsystems is that of an AC-coupled system, where repetitive signals decayuntil there is an equal area above and below the original base line. Asmagnitude levels of the monitored event fluctuate, the output remainsstabilized around the base line with the positive and negative areas ofthe curve remaining equal.

[0083] The pressure sensors 218-221 described herein may be any type ofpressure sensor, capable of measuring the unsteady (or ac or dynamic )pressures within a pipe, such as piezoelectric, optical, thermal,capacitive, inductive, resistive (e.g., Wheatstone bridge),accelerometers (or geophones), velocity measuring devices, displacementmeasuring devices, etc. If optical pressure sensors are used, thesensors 218-221 may be Bragg grating based pressure sensors, such asthat described in U.S. patent application Ser. No. 08/925,598, entitled“High Sensitivity Fiber Optic Pressure Sensor For Use In HarshEnvironments”, filed Sep. 8, 1997, now U.S. Pat. No. 6,016,702.Alternatively, the sensors 23 may be electrical or optical strain gagesattached to or embedded in the outer or inner wall of the pipe whichmeasure pipe wall strain, including microphones, hydrophones, or anyother sensor capable of measuring the unsteady pressures within the pipe214. In an embodiment of the present invention that utilizes fiberoptics as the pressure sensors218-221, they may be connectedindividually or may be multiplexed along one or more optical fibersusing wavelength division multiplexing (WDM), time division multiplexing(TDM), or any other optical multiplexing techniques.

[0084] A piezo-electronic pressure transducer may be used (oralternatively even a common strain gage may be used) as one or more ofthe pressure sensors 218-221, and it may measure the unsteady (ordynamic or ac) pressure variations Pin inside the pipe 214 by measuringthe pressure levels (or for the strain gage, the elastic expansion andcontraction of the diameter of the pipe 214. In an embodiment of thepresent invention the sensors 218-221 comprise pressure sensorsmanufactured by PCB Piezotronics. In one pressure sensor there areintegrated circuit piezoelectric voltage mode-type sensors that featurebuilt-in microelectronic amplifiers, and convert the high-impedancecharge into a low-impedance voltage output. Specifically, a Model 106Bmanufactured by PCB Piezotronics is used which is a high sensitivity,acceleration compensated integrated circuit piezoelectric quartzpressure sensor suitable for measuring low pressure acoustic phenomenain hydraulic and pneumatic systems. It has the unique capability tomeasure small pressure changes of less than 0.001 psi under high staticconditions. The 106B has a 300 mV/psi sensitivity and a resolution of 91dB (0.0001 psi).

[0085] For any of the embodiments described herein, the pressuresensors, including electrical strain gages, optical fibers and/orgratings among others as described herein, may be attached to the pipeby adhesive, glue, epoxy, tape or other suitable attachment means toensure suitable contact between the sensor and the pipe 212. The sensorsmay alternatively be removable or permanently attached via knownmechanical techniques such as mechanical fastener, spring loaded,clamped, clam shell arrangement, strapping or other equivalents.Alternatively, the strain gages, including optical fibers and/orgratings, may be embedded in a composite pipe. If desired, for certainapplications, the gratings may be detached from (or strain oracoustically isolated from) the pipe 212 if desired.

[0086] It is also within the scope of the present invention that anyother strain sensing technique may be used to measure the variations instrain in the pipe, such as highly sensitive piezoelectric, electronicor electric, strain gages attached to or embedded in the pipe 212.

[0087] While the sonar-based flow meter using an array of sensors tomeasure the speed of sound of an acoustic wave propagating through themixture, one will appreciate that any means for measuring the speed ofsound of the acoustic wave may used to determine the entrained airvolume fraction of the mixture/fluid.

[0088] It should be understood that, unless stated otherwise herein, anyof the features, characteristics, alternatives or modificationsdescribed regarding a particular embodiment herein may also be applied,used, or incorporated with any other embodiment described herein.

[0089] Although the invention has been described and illustrated withrespect to exemplary embodiments thereof, the foregoing and variousother additions and omissions may be made therein without departing fromthe spirit and scope of the present invention.

What is claimed is:
 1. A device having a first module arranged inrelation to a process line for providing a first signal containinginformation about sensed entrained air/gas in a fluid or process mixtureflowing in the process line at a process line pressure, the devicecomprising: a bleed line coupled to the process line for bleeding aportion of the fluid or process mixture from the process line at a bleedline pressure that is lower than the process pressure; a second modulearranged in relation to the bleed line, for providing a second signalcontaining information about sensed bleed line entrained air/gas in thefluid or process mixture flowing in the bleed line; and a third moduleresponsive to the first signal and the second signal, for providing athird signal containing information about a dissolved air/gas flowing inthe process line based on a difference between the sensed entrainedair/gas and the sensed bleed line entrained air/gas.
 2. A deviceaccording to claim 1, wherein the first module is a primary process lineentrained air measurement module.
 3. A device according to claim 1,wherein the first module includes an array of sensors that measures thespeed of sound propagating through the fluid or process mixture flowingwithin the process line and determines the entrained air based on ameasurement using the speed of sound.
 4. A device according to claim 1,wherein the second module is a bleed line entrained air measurementmodule.
 5. A device according to claim 1, wherein the second moduleincludes an array of sensors that measures the speed of soundpropagating through the fluid or process mixture flowing within thebleed line and determines the bleed line entrained air based on ameasurement using the speed of sound.
 6. A device according to claim 1,wherein the third module is a dissolved air determination processormodule.
 7. A device according to claim 1, wherein the bleed line isre-coupled to the process line to recirculate the portion of the fluidor process mixture bled from the bleed line.
 8. A device according toclaim 1, wherein the device includes a boost pump for re-coupling thebleed line to the process line to re-pressurize and reinject the portionof the fluid or process mixture bled back to the process line.
 9. Adevice according to claim 1, wherein the device includes a bleed linecontrol module for controlling the bleeding off of the portion of thefluid or process mixture from the process line and the reinjection ofthe same back to the process line.
 10. A device according to claim 1,wherein the portion of the fluid or process mixture from the processline is bled off either continuously or periodically.
 11. A deviceaccording to claim 1, wherein the bleed line and flow rates are sized tominimize the amount of stock bleed off while maintained sufficientlyhigh flow rates to maintain sufficiently homogenous flow within abled-off liquid test section such that a measured gas volume fractionwithin the bleed line is representative of the amount of gas dissolvedin the fluid or process mixture.
 12. A device according to claim 1,wherein sufficiently high velocities are maintained to avoid problemsassociated with stratification of the mixture and the problemsassociated with either the liquid of gas phases “holding up” in theprocess pipe.
 13. A device according to claim 1, wherein the deviceincludes a controller module for controlling the first module, thesecond module and the third module.
 14. A device for measurement ofentrained and dissolved gases in a fluid or process mixture flowing in aprimary process line having a process pressure, the device comprising: afirst entrained air measurement module arranged in relation to theprimary process line, for sensing entrained air in the fluid or processmixture and providing a first entrained air measurement module signalcontaining information about sensed primary process line entrained air;a bleed line coupled to the primary process line for bleeding a portionof the fluid or process mixture from the primary process line at a bleedline pressure that is lower than the process pressure; a secondentrained air measurement module arranged in relation to the bleed line,for sensing entrained air in the fluid or process mixture in the bleedline, and providing a second entrained air measurement module signalcontaining information about sensed bleed line entrained air; and adissolved air/gas determination processor module responsive to the firstentrained air measurement module signal and the second entrained airmeasurement module signal, for providing a dissolved air/gasdetermination processor module signal containing information about adissolved air/gas in the fluid or process mixture flowing in the primaryprocess line based on a difference between the sensed primary processline entrained air and the sensed bleed line entrained air.
 15. A deviceaccording to claim 14, wherein the primary process line entrained airmeasurement module includes an array of sensors that measures the speedof sound propagating through the fluid or process mixture flowing in theprocess line and determines the entrained air based on a measurementusing the speed of sound.
 16. A device according to claim 14, whereinthe bleed line entrained air measurement module includes an array ofsensors that measures the speed of sound propagating through the fluidor process mixture flowing in the bleed line and determines the bleedline entrained air based on a measurement using the speed of sound. 17.A device according to claim 14, wherein the bleed line is re-coupled tothe primary process line via a boost pump to reinject the fluid orprocess mixture bled back into the primary process line.
 18. A deviceaccording to claim 14, wherein the device includes a bleed line controlmodule for controlling the bleeding off of the portion of the fluid orprocess mixture from the process line and the reinjection orrecirculation of the same back to the primary process line.
 19. A methodfor measuring entrained and dissolved gas in a fluid or process mixtureflowing in the process line at a process line pressure, comprising thesteps of: measuring a first entrained gas in the fluid or processmixture flowing in the process line and providing a first signalcontaining information about the same; bleeding a portion of the fluidor process mixture from the process line into bleed line having a bleedline pressure that is lower than the process pressure; measuring asecond entrained gas in the fluid or process mixture flowing in thebleed line providing a second signal containing information about thesame; and responding to the first signal and the second signal, anddetermining a dissolved air/gas flowing in the process line based on adifference between the first entrained air/gas and the second bleed lineentrained air/gas.
 20. A method according to claim 19, wherein the stepmeasuring the first entrained gas includes arranging a first modulehaving an array of sensor in relation to the process line fordetermining the first entrained gas based on a speed of soundmeasurement.
 21. A method according to claim 19, wherein the stepmeasuring the second entrained gas includes arranging a second modulehaving a corresponding array of sensor in relation to the bleed line fordetermining the second entrained gas based on a corresponding speed ofsound measurement.